The frequency of tropical precipitating clouds as observed by the TRMM PR and ICESat/GLAS

Abstract

Convective clouds in the tropics can be grouped into three categories: shallow clouds with cloud-top heights near 2 km above the surface, mid-level congestus clouds with tops near the 0°C level, and deep convective clouds capped by the tropopause. This trimodal distribution is visible in cloud data from the Geoscience Laser Altimeter System (GLAS), carried aboard the Ice, Cloud, and land Elevation Satellite (ICESat), as well as in precipitation data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Fractional areal coverage (FAC) data is calculated at each of the three levels to describe how often optically thick clouds or precipitation are seen at each level. By dividing the FAC of TRMM PR-observed precipitation by the FAC of thick GLAS/ICESat-observed clouds, the fraction of clouds that are precipitating is derived. The tropical mean precipitating cloud fraction is low: 3.7% for shallow clouds, 6.5% for mid-level clouds, and 24.1% for deep clouds. On a regional basis, the FAC maps created in this study show interesting trends. The presence of nonphysical answers in the PCF graphs, however, suggest that greater study with more precise instruments is needed to properly understand the true precipitating cloud fraction of the tropical atmosphere.